This invention relates to methods for laser texturing magnetic disks and the resulting disks.
It is known in the art to manufacture magnetic disks using the following method.
1. First, a NiP alloy layer is electroless plated onto an Al alloy substrate. This NiP alloy is hard, and prevents the disk from being dented if it subsequently strikes a read-write head during use.
2. The NiP alloy layer is then polished.
3. The NiP alloy layer is then laser textured at a region of the disk known as the contact-start-stop, or CSS, zone. This zone is where a read-write head rests when the disk is not in use. Of importance, laser texturing reduces static friction (xe2x80x9cstictionxe2x80x9d) and dynamic friction between the magnetic disk and the read-write head when the disk drive is turned on or off. Reducing stiction and friction is important for increasing the useful life of the magnetic disk.
4. An underlayer (e.g. Cr or NiP), a magnetic Co alloy, and a protective overcoat (e.g. zirconia or hydrogenated carbon) are then sputtered, in that order, onto the laser textured NiP layer.
5. A lubricant is then applied to the protective overcoat. After fabrication, the disk is incorporated into a hard disk drive.
The laser texture is applied to the disk by applying laser pulses to the Nip alloy layer while the substrate is spinning. The laser pulses result in the formation of laser bumps, which can be either xe2x80x9cridge-shapedxe2x80x9d or xe2x80x9csombrero-shaped,xe2x80x9d depending on various factors such as the laser pulse parameters. (Ridge and sombrero-shaped bumps are discussed, for example, In U.S. Pat. No. 5,741,560, issued to Ross, incorporated herein by reference.)
The bumps are typically arranged in a spiral pattern on the CSS zone of the disk surface. When the disk drive is off, the read-write head rests on the laser bumps in the CSS zone. When the disk drive is first turned on, the read-write head drags across the surface of the CSS zone until the disk is spinning fast enough to create an air bearing for the head to xe2x80x9cflyxe2x80x9d on. The laser bumps reduce static and dynamic friction between the read-write head and the disk during this process. When the disk drive is turned off, the read-write head is positioned over the CSS zone, and the disk rotation decelerates until the read-write head comes into contact with the CSS zone. Eventually, the read-write head comes to rest on the CSS zone when the disk stops spinning.
Regardless of which shape laser texture bumps are used, if the bumps are spaced at a constant distance, the bumps may cause the read-write head to vibrate when the head takes off and lands. If the read-write head is caused to vibrate at a natural resonant frequency of the read-write head mechanism, these vibrations may be exacerbated. There are several parts of the read-write mechanism that have their own natural resonant frequency. For example, the air bearing itself may have a natural resonant frequency at about 100 KHz. The slider body for a glide head having a 50% form factor (e.g. a slider having a piezoelectric transducer coupled thereto that is part of test apparatus) may resonate at about 250 KHz. A slider body for a read-write head having a 50% form factor may have a natural resonant frequency at 750 KHz. (The term xe2x80x9c50% form factor refers to the slider size, i.e. 2 mm long by 1.6 mm wide by 0.433 mm high.)
The read-write head mechanically interacts with the bumps while the disk rotation is accelerating or decelerating. Therefore, at certain times the disk will be spinning at a velocity such that if the bumps are regularly spaced, they provide mechanical excitation to the read-write head at a frequency close to one of the natural resonant frequencies of the read-write head mechanism. While there is debate about the specific effects that read-write head vibration has on a disk drive, generally there is agreement that reducing or eliminating these vibrations is desirable. See, for example, Yao et al., xe2x80x9cHead-Disc Dynamics of Low Resonance Laser Texturesxe2x80x94A Spectrogram Analysisxe2x80x9d, IEEE Trans. On Magnetics, Vol. 34, No. 4, July 1998, page 1699-1701.
Since the laser bumps can cause vibrations in the read-write head, it has been suggested that the vibrational excitation of the read-write head can be reduced by altering the position and spacing of the laser bumps. One technique for varying the position and spacing of the laser bumps in the plated NiP alloy layer is to vary the laser pulse frequency. In particular, by randomly varying the laser frequency, one could vary the bump spacing. Unfortunately, without using special techniques, this would compromise the stability of the energy per pulse of the laser. Such stability is critical to making consistent laser bumps of a controlled height.
Another technique for varying the position of the laser bumps would be to block selected laser pulses with a solid-state shutter. This would produce again a spiral zone pattern, but with variable length xe2x80x9cgapsxe2x80x9d where bumps would be missing. Unfortunately, these gaps in the texture pattern would be multiples of one fundamental spacing distance based on the laser pulse frequency. This means that the pattern would still be capable of exciting vibration in the read-write head at the natural resonant frequency of the read-write head mechanism.
A method for laser texturing a substrate in accordance with our invention comprises the step of rotating a substrate, varying the rotational velocity of the substrate, and applying laser pulses to at least a portion of the substrate while the substrate is being rotated. In one embodiment, the rotational velocity of the substrate is varied in accordance with a substantially periodic function. In one embodiment, the rotational velocity of the substrate is varied sinusoidally or in accordance with a sawtooth function. After texturing, a magnetic layer is deposited on the substrate to form a magnetic disk. (Typically, other layers, such as an underlayer and a protective overcoat are also deposited on the substrate. The underlayer is formed between the substrate and the magnetic layer, and the protective overcoat is formed over the magnetic layer.)
We have discovered that by varying the position of the laser bumps in this manner, we can sharply reduce the vibrational energy imparted to a read-write head by the laser bumps when the read-write head takes off from or lands on a magnetic disk.
In one embodiment, during laser texturing, the substrate spins at a mean velocity of 3200 RPM. The velocity of the substrate is varied by 300 RPM about this mean velocity. In other words, the rotational velocity varies between 2900 and 3500 RPM. During this process, laser pulses of a constant frequency are applied to the substrate, thereby forming a laser bump pattern in which the spacing of the laser bumps varies periodically. In one embodiment, the laser bumps are arranged in a spiral. In another embodiment, the laser bumps are arranged in concentric circles.
In accordance with another aspect of our invention, a magnetic disk comprises a pattern of texture bumps in which the spacing of the bumps is periodically varied in the circumferential direction. This pattern of laser bumps is neither random, nor are the bumps regularly spaced.